The idea behind credit cards that offer airline rewards is simple enough: The more you spend on the card, the more free airline miles you get.

But the U.S. Mint recently broke up a scheme by cardholders who figured out a way to get money for nothing and rewards for free.

Under the scheme, people used their credit cards to purchase dollar coins from the U.S. Mint. The cardholders then deposited the coins in the bank to pay off the credit card balance.

In the end, the cardholders were out no money but their credit card registered thousands of dollars worth of spending, and thus they earned thousands of rewards points or miles from the airlines.

"It's not the right thing to do," said Tom Jurkowsky, a spokesman for the U.S. Mint. "It's not what the program was meant to do."

Since November, the U.S. Mint has written letters to about 75 cardholders who may have tried the scheme to collect airline reward points.

The trouble began in June 2008 when the U.S. Mint launched a "direct ship" program to sell dollar coins directly to the general public in hopes of increasing the use of the coins, which last longer than paper currency.

Under the program, individuals could buy a maximum of 500 coins of each of five presidential dollars created by the mint (Washington, Adams, Jefferson, Madison and Jackson). But the mint set no limit for coins emblazoned with an image of an American Indian figure. The mint allowed people to buy the coins with credit cards and offered free shipping in the continental United States.

Several savvy frequent fliers got the idea to buy the coins with credit cards to accumulate reward points and then pay off the credit card balance after depositing the coins at a bank.

Word spread about the scheme on Internet blogs, such as Flyertalk.com. The Wall Street Journal, which recently broke the story, quoted a frequent flier who identified himself as Mr. Pickles and claimed he bought $800,000 in coins with his credit cards to jack up his rewards point total.

He told the Journal that he pulled off the scheme by using several banks and numerous credit cards.

Although no laws were broken, officials at the U.S. Mint said the deal conflicted with the intent of the program, which was to circulate the coins throughout the economy.

Under the scheme, the coins were deposited directly into banks. Jurkowsky said the mint already has a program to distribute the dollar coins in bulk to banks.

Since the program began, the mint recorded about 40,000 "direct ship" coin orders. Jurkowsky said the mint believes fewer than 1 percent of those orders may have been made to generate airline miles.

In November and December, the mint sent letters to 75 people who used credit cards to buy large orders of coins, asking them to explain their unusual purchases. Jurkowsky said some of the people who responded to the letters had legitimate explanations, such as owners of Laundromats who need the coins to operate washers and dryers.

The mint is now making a permanent fix to the problem,

Jurkowsky said. In the future, credit card purchases of the coins will be recorded as a cash advance rather than a credit card purchase. Credit cards typically do not give reward points for cash advances.